The coating of electrically conductive substrates by an electrocoating process is a well-known and important industrial process. The electrocoating of primers to substrates is widely used in the automotive industry. In this process, a conductive article, such as an automobile body or an automobile part, is immersed in a bath comprising an aqueous emulsion of film forming polymer and acts as an electrode in the electrocoating process. An electric current is passed between the article and a counter-electrode in electrical contact with the aqueous emulsion, until a desired coating is deposited on the article. In a cathodic electrocoating process, the article to be coated is the cathode and the counter-electrode is the anode.
Film forming polymer compositions used in the bath of a typical cathodic electrocoating process are well known in the art. These polymers are typically made from polyepoxides which have been chain extended using bisphenol compounds. The chain extended polyepoxides can then be reacted with amines to form an epoxy amine adduct. These polymers are blended with a crosslinking agent and then neutralized with an acid to form a water emulsion, which is usually referred to as a principal emulsion.
The principal emulsion can be combined with a pigment paste, coalescent solvents, water, and other additives to form the electrocoating composition. The composition is placed in an insulated tank containing the anode. The article to be coated is the cathode and is placed in a tank containing the electrocoating composition. An electrical current is applied to the system and a layer of the electrocoating composition is deposited onto the article. The thickness of the applied layer of electrocoating composition that is deposited on the article is a function of, for example, the bath characteristics, the electrical operating characteristics and the immersion time.
The resulting coated article is removed from the bath after a period of time and is rinsed with deionized water. The coating on the article can then be cured, typically in an oven, at sufficient temperature to produce a crosslinked finish on the article.
Cathodic electrocoating compositions, resin compositions, coating baths and cathodic processes are disclosed in Jarabek et al U.S. Pat. No. 3,922,253 issued Nov. 25, 1975; Wismer et al U.S. Pat. No. 4,419,467 issued Dec. 6, 1983; Belanger U.S. Pat. No. 4,137,140 issued Jan. 30, 1979 and Wismer et al U.S. Pat. No. 4,468,307 issued Aug. 25, 1984.
A continuing problem with cathodic electrocoating compositions has been the presence of craters in the cured finish. A number of anticrater agents have been used in the past to eliminate craters. However, the presence of conventional anticrater agents in electrocoating compositions has had a negative impact on the adhesion of subsequent coating layers applied thereto, such as automotive PVC sealers used for sealing joints and primer surfacers, particularly where the electrocoating film has been cured in an oven without the presence NOx (nitrogen oxides), such as in an indirect gas or electric oven. There is a continuing need for electrocoating compositions that can produce crater-free, smooth and even finishes that do not adversely affecting the adhesion of coatings that are subsequently applied to the electrocoated substrate.